1. Field of the Invention
The present invention relates to a pressure control actuator and specifically to an automotive brake control system with a fluid-pressure control actuator which is suitably utilized for both an anti-skid brake control and a traction control.
2. Description of the Prior Art
Recently, there have been proposed and developed various pressure control actuators, such as a fluid-pressure operated pressure control actuator and a powered piston-type pressure control actuator. An anti-lock brake system equipped with such a conventional powered piston-type pressure control actuator has been disclosed in Japanese Patent First Publication (Tokkai Heisei) No. 3-167058 (corresponding to U.S. Pat. application No. 07/438,174, filed on Nov. 16, 1989. The above-mentioned prior-art piston-type pressure control actuator is shown in FIG. 7. One set of the powered piston-type pressure control actuator is provided at each of vehicle wheel cylinders.
Referring now to FIG. 7, the conventional anti-lock brake system includes a first shut-off valve 53 connected through its first port K to an outlet port of a master cylinder 52 and a pressure regulating cylinder unit (actuator) 54 connected through its upper variable-volume chamber 55 to a second port L of the first shut-off valve 53. In a conventional manner, the master cylinder 52 produces a master-cylinder pressure based on an amount of depression of a brake pedal 51. The pressure regulating cylinder unit 54 includes a substantially cylindrical housing 56 having three-stepped cylindrical bores, namely an upper bore 56a having a relatively large inside diameter, an intermediate bore 56b having a relatively medium inside diameter and a lower bore 56c having a relatively small inside diameter. The cylindrical housing 56 slidably encloses a first piston 58 in the upper bore 56a. The upper bore 56a defines the upper variable-volume chamber 55 in conjunction with the first piston 58. The first piston 58 is normally biased in a lowermost spring-set position by the bias of a return spring 65, so that the bottom of the piston 58 abuts a stepped portion between the two consecutive bores 56a and 56b. The cylinder unit 54 includes a guide cylinder 61 disposed in the intermediate bore 56b. The guide cylinder 61 slidably encloses therein a second push-rod type piston 59, such that the upper end of the second piston 59 is abutable with the bottom surface of the first piston 58. A lower variable-volume chamber 60 is defined between the first piston 58 and the guide cylinder 61. Reference numerals 57, 62 and 63 denote oil seals to prevent oil leakage between the inner wall of the housing 56 and the outer periphery of the first piston 58, between the inner periphery of the guide cylinder 61 and the outer periphery of the second piston 59, and the outer periphery of the guide cylinder 61 and the inner wall of the housing 56, respectively. The first piston 58 operably encloses therein a check valve 64 to permit fluid flow from the lower chamber 60 to the upper chamber 55 and to prevent back-flow therethrough back to the lower chamber 60. The check valve 64 is opened by means of an upper semi-spherical projection 59a integrally formed on the upper flat end of the second piston 59. An additional spring 66 is operably disposed in the lower chamber 60 to bias the first piston 58 upwards and to bias the guide cylinder 61 downwards so that the guide cylinder 61 is maintained in its lowermost position. As compared with the spring 65, a spring constant of the additional spring 66 is set to a smaller value so that the first piston 58 is normally maintained in a neutral position (the lowermost position shown in FIG. 7). A motor-driven thrust generator 70 is provided at the lower end of the second piston 59. The thrust generator 70 comprises a reversible motor 67, a gear train 68, a worm shaft 69, and a ball guide nut 73. The worm shaft 69, the ball guide nut 73, the first and second pistons 58 and 59, the two springs 65 and 66, and the cylindrical housing 56 are axially aligned with each other. The worm shaft 69 engages the ball guide nut 73 by way of recirculating balls in order to permit both an axial movement and a rotational movement of the guide nut 73. Actually, since the guide nut 73 is splined to the bore 56c of the housing 56, rotation of the worm shaft 69 allows only the axial movement of the guide nut 73. The free end portion of the worm shaft 69 is operably enclosed in the cylindrical hollow 59b of the second piston 59, while the upper flat end of the guide nut 73 is rigidly connected to the flanged portion of the second piston 59C for axial movement therewith. The conventional anti-lock brake system illustrated in FIG. 7 includes a second shut-off valve 72 which is fluidly disposed between the second port L of the first shut-off valve 53 and an inlet-and-outlet port of a wheel cylinder 71. Each of the previously-noted first shut-off valve 53 and the second shut-off valve is traditionally comprised of a normally-open-type shut-off valve, such as a normally-open-type two-port two-position electromagnetic solenoid valve. The port of the wheel cylinder 71 is also connected through a slotted fluid passage 74 of the housing 56 to the lower chamber 60. Reference numeral 75 designates a brake controller which executes either an anti-skid brake control generally abbreviated as an "ABS control" or a traction control generally abbreviated as a "TCS control", depending on the vehicle driving condition. The controller 75 is connected to a wheel speed sensor 76 and a vehicle speed sensor (not shown) to derive a slippage of the road wheel on the basis of signal values generated by the sensors and to execute either the ABS control or the TCS control, based on the comparison results of the slippage to a threshold value. The controller 75 is connected to the two shut-off valves 53 and 72 to properly open and close the respective valve and further connected to the reversible motor 67 of the thrust generator 70 to generate axial thrust of the second piston 59 by clockwise or counterclockwise rotational movement of the motor 67.
During the ABS control, the controller 75 outputs a control signal to the second shut-off valve 72 to fully close the valve 72. Additionally, the first shut-off valve is kept opened. Upon termination of the shut-off operation of the second valve 72, the controller 75 drives the motor 67 to produce a downward displacement of the second piston 59, with the result that the check valve 64 is fully closed and the volume of the lower chamber 60 is enlarged, thereby resulting in a decrease in the wheel-cylinder pressure. In this manner, the braking force applied to the road wheel is effectively reduced at the ABS pressure reduction mode.
During the TCS control, the controller 75 outputs a control signal to the first shut-off valve 53 to fully close the valve 53. The second valve 72 is held opened. Upon termination of the shut-off operation of the first valve 53, the controller 75 drives the motor 67 to create an upward displacement of the second piston 59, with the result that the upper end of the second piston 59 is further projected into the second chamber 60 and consequently the first piston 58 is pushed upwards. The sum of the volume of the upper variable-volume chamber 55 and the volume of the lower variable-volume chamber 60 is decreased by a further projected portion of the piston 59. Accordingly, the wheel-cylinder pressure in the wheel cylinder 71 which is fluidly connected through the second valve 72 to the two chambers 55 and 60 is increased, thereby causing a properly increased braking force.
The conventional anti-lock brake system with the previously-noted pressure regulating actuator suffers from the drawback that two electromagnetic solenoid-type shut-off valves are required to control fluid flow to each wheel cylinder. In other words, the conventional anti-lock brake system requires at least eight shut-off valves per car. This results in high production costs. Additionally, the pressure regulating actuator incorporated in the conventional anti-lock brake system requires three oil seals 57, 62 and 63, for the purpose of preventing of oil leakage. Owing to a relatively large number of oil seals, a reliability to oil leakage is low.
Furthermore, the conventional anti-lock brake system has some problems concerning unstable control operations, during traction control, during normal braking, and during anti-skid brake control. For example, when the brake pedal is depressed during the traction control, the closed shut-off valve 53 is not shifted to the open position soon. In this case, the valve 53 is recovered to the full open position with a slight delay time. Such a delay time of valve-shift causes a slightly delayed brake timing.
During normal braking, it is necessary to prevent the second piston 59 from moving downwards by the master-cylinder pressure and to maintain the second piston 59 in its neutral position by way of electromagnetic braking action on the motor 67, thereby avoiding damper action of the piston 59 and resulting in effective braking effect. Such an electromagnetic brake is very expensive. Assuming that the above-mentioned electromagnetic brake is damaged under the open condition of the valves 53 and 72, there is a tendency for the second piston 59 to move downwards owing to the master-cylinder pressure. The downward stroke of the piston 59 prevents an increase in the wheel-cylinder pressure. In this case, an effective braking effect is not obtained.
In addition to the above, the prior-art pressure regulating actuator suffers from the drawback that it is difficult to precisely return the second piston 59 in the neutral position with a proper current control. For instance, In the event that the ABS control is terminated when the vehicle is running on a low frictional road surface such as wet or icy roads, the second shut-off valve 72 is shifted from the closed position to the open position, under depression of the brake pedal. Under this condition, assuming that the wheel-cylinder pressure is zero, the master-cylinder pressure tends to push the second piston 59 in the axially downward direction. To avoid the downward stroke of the piston 59, the motor 67 is driven by an extremely small current to such a degree that the piston 59 is not quickly but moderately moved upward. In order to achieve more accurate approach to the neutral position, the second piston 59 must be gradually and slowly returned to the neutral position. Thus, it takes a relatively long time necessary for recovery to the neutral position. Conversely, in the event that the ABS control is terminated when the vehicle is running on a high frictional road such as dry pavements, the wheel-cylinder pressure often remains at a pressure level above zero. In this case, since the second piston 59 is positioned in a lower position than the neutral position and in addition the upper end of the second piston 59 receives the wheel-cylinder pressure, a suitable current must be applied to the motor 67 such that the upper end of the second piston 59 abuts the bottom of the first piston 58 with relatively moderate upward movement of the piston 59. In practice, it is difficult to precisely return the piston 59 in the neutral position, since a proper current value necessary for return to the neutral position cannot be derived without input information, such as an actual master-cylinder pressure, an actual wheel-cylinder pressure, and an actual position of the piston 59. In the event that the vehicle travels on high and low friction roads alternately with depression of the brake pedal, the ABS control operation will be repeatedly executed. Assuming that the second piston 59 is positioned in its neutral position when the first ABS control is initiated, the first cycle of a pressure-reduction ABS control is certainly executed in accordance with a downward stroke started from the neutral position. However, upon initiation of the second ABS control, there is a great possibility that the second piston 59 offsets from the neutral position, owing to improper current control. For example, supposing that the second piston 59 is positioned at a lower level than the neutral position just before initiation of the second ABS control, the pressure regulating cylinder unit 54 cannot provide a downward stroke enough for pressure reduction during the second ABS control. In contrast, supposing that the second piston 59 is positioned at a higher level than the neutral position just before initiation of the second ABS control, the pressure regulating cylinder unit 54 cannot provide an optimal timing of pressure reduction, because the system takes a long time until the piston 59 reaches to the neutral position according to its downward stroke, and thus the check valve 64 is not yet closed.
On the other hand, when the brake pedal is depressed upon termination of the TCS control, the respective pistons 58 and 59 may not be completely shifted to their neutral positions, due to the delay time. Under these conditions, in the event that the ABS control is initiated, there are some problems, such as an unsatisfactory downward stroke or an undesired timing for pressure reduction, for the reasons indicated above.